Application of infrared heating in low temperature rapid curing(1)

1 Preface

Under the general trend of global green and environmental protection production, General Secretary Xi Jinping solemnly declared at the 75th session of the United Nations General Assembly: "China will peak carbon emissions before 2030 and achieve carbon neutrality by 2060"
.
Since then, China has moved towards the era
of "30/60 dual carbon" of carbon peak and carbon neutrality.

Although powder coating has the "5E" advantages of Efficient (high efficiency), Energy Saving, Environmental Friendly, Excellent finishing, Enabling (wide adaptability), coating online coatingol.
com
.
However, in the spraying construction of powder, the heating and curing process consumes a lot of manpower and material resources, and there is still a lot of economic space for improvement
.
If the idle employees are fully utilized when this process is heated up, and the heating rate of baking is increased, and the powder curing time is shortened, energy can also be saved and production efficiency
can be improved.
It can alleviate the problems of hot hands such as difficulty in recruiting workers and staggered peak electricity consumption, and also responds to the call of "30/60 dual carbon"
.

2 The connection and difference between low-temperature curing and fast-curing powder

The standard curing temperature of powder coating is between 180 °C ~ 200 °C, if the curing temperature is significantly lower than the standard curing temperature of the powder, it is called low-temperature curing powder
.
Some heat-sensitive substrates and electronic components are sensitive to temperature, and high temperatures can damage them, so the coating selected for decoration and protection should be suitable for low-temperature curing
.
For example, MDF special powder and electronic encapsulation special powder can be cured at 120 °C ~ 140 °C / 10min ~ 30min, which is a common low temperature curing powder
.

The standard curing time of powder coating is between 10min~15min, if the curing time is significantly shorter than the standard curing time, the powder is called fast-curing powder
.
Epoxy steel bar special powder and coil steel special powder are common fast-curing powders
.
The special powder for epoxy steel bar is particularly reactive and can be completely cured
at 230°C/9s.

Low temperature curing is not equal to fast curing, low temperature curing is for temperature; Fast curing is a matter of
time.
However, low-temperature curing and rapid curing are linked
.
The curing temperature increases, the reactivity is enhanced, and the curing time required becomes shorter
.
Conversely, the curing temperature decreases, the reactivity is weakened, and the curing time required becomes longer
.
For example, a coil of steel special powder cured at 130 °C/5min in the laboratory can meet the curing conditions
of 240 °C~260 °C/30s in the actual production line.

3 Obstacles to the development of low-temperature cured powders

Low-temperature curing powders can generally be divided into two categories
: heat curing and UV (ultraviolet) curing.
Heat curing is cross-linking curing by heating to a certain temperature, which is well known; UV curing is the powder after being heated and flattened, and then cured
by UV radiation.
If there is no UV radiation, the powder will not cure
even when heated to very high temperatures.

The heat-curing powder flattening and curing process are continuous, which is the curing mechanism of polymerization
.
The softening point of the powder base resin is between 87 °C ~ 120 °C, and the extrusion processing temperature must be slightly higher
than the softening point.
However, the extrusion temperature is not far from the curing temperature, and the processing process is prone to collided particles
.
Moreover, low-temperature heat-curing powder requires low temperature storage and transportation, otherwise, drawbacks such as powder agglomeration or slow reaction will occur, which will bring inconvenience
to the application.

UV curing powder flattening and curing process separation is the curing mechanism of chain polymerization
.
The curing process consists of
primitive reaction series/parallels such as chain initiation, chain growth, chain termination, and chain transfer.
The schematic diagram of melting and solidification is shown in Figure 1
.
UV-curable powders avoid the temperature effects
of heat-curing extrusion processing, storage and transportation.
However, some photoinitiators have problems such as
migration and reproductive toxicity.
For example, photoinitiators containing aromatic amine groups are carcinogenic, and photoinitiator 907 has reproductive toxicity and developmental toxicity, which are listed as toxic and harmful substances
by EU REACH and North American TSCA.
This is contrary to
the direction of the development of "green coatings".
To a certain extent, it affects the development of
UV-cured powder.
In addition, the resins and equipment used for UV-curable powders are produced by very few domestic enterprises, which also hinders the development of
UV-curable powders.

Figure 1 Schematic diagram of the melting and curing mechanism of UV-cured powder

Therefore, at present, the domestic low-temperature curing powder is still mainly heat cured
.

4 Several ways of heat transfer

Fast curing mainly emphasizes time efficiency
at the same temperature.
To reach a certain temperature, there are three heat transfer methods
: conduction, convection and radiation.
For example, these three heat transfer methods are included when boiling water, as shown in Figure 2
below.

Figure 2 Schematic diagram of conduction, convection and radiation

When the parts of the object do not undergo relative displacement, the heat energy transfer generated by the thermal movement of microscopic particles such as molecules, atoms and free electrons is called conduction, referred to as heat
conduction.
The conduction effect of metal conductors is good, and the conduction effect of air is poor
.

Convection refers to the heat transfer process
caused by the relative displacement of various parts of the fluid caused by the macroscopic motion of the fluid, and the mixing of hot and cold fluids.
Convection must be accompanied by conduction
.
Water is much
stronger than convective heat transfer from air.

The way an object transmits energy through electromagnetic waves is called radiation
.
The method of transferring heat by electromagnetic waves is called radiative heat
transfer.
The two methods of heat transfer, heat conduction and convection, can only be achieved in the presence of matter, and thermal radiation can be transferred in a vacuum, and in fact, the transfer of radiant energy in a vacuum is more efficient
.

The picture is above the thermodynamic temperature scale "absolute zero" 0K (Kelvin temperature K = °C + 273.
15), and everything can emit infrared light
.
The emission of infrared rays
is achieved by changing the dipole moment.
Calculated
according to Wilhelm Wien's Wien law of displacement.
The correspondence between the operating temperature range and the radiation peak λmax of various commercially available infrared emitters is shown in Table 1
.

Table 1 Correspondence between source temperature and peak radiation wavelength

Source: 2021 China Powder Coating and Coating Conference Journal Li Liangjian Xie Pinxiong/Foshan Nanhai Jiaduocai Powder Coating Co.
, Ltd